U.S. patent number 6,233,085 [Application Number 09/420,391] was granted by the patent office on 2001-05-15 for apparatus, method, and computer program product for controlling an interferromic phased array.
This patent grant is currently assigned to The Boeing Company. Invention is credited to Bartley C. Johnson.
United States Patent |
6,233,085 |
Johnson |
May 15, 2001 |
Apparatus, method, and computer program product for controlling an
interferromic phased array
Abstract
The present invention provides several apparatus, methods, and
computer program products for phase shifts in an optical signal
generated by an optical device. The present invention includes a
optical phase modulator for altering the phase of the optical
signals output by an optical device. The present invention also
includes a detector assembly that receives an interference signal
generated by an optical interference of a power signal and a
reference signal. The detector assembly generates a lockin signal
representing the optical phase difference between the power signal
and the reference signal. Connected to both the detector assembly
and the optical phase detector is a processor. The processor
receives the lockin signal from the detector assembly and compares
the lockin signal to at least one predetermined set point value.
The predetermined set point value represents either a desired
maximum lockin signal value or a range of acceptable lockin values.
If the lockin signal either exceeds the set point value or is not
within a range of lockin signal values defined by the predetermined
set point value, the processor combines the lockin signal with a
step waveform and generates a combined feedback signal having a
value within desired operation parameters of the optical phase
modulator. In one embodiment of the invention, the processor
combines the lockin signal with an exponential step function having
a step portion of 2.pi.n.
Inventors: |
Johnson; Bartley C. (Clayton,
MO) |
Assignee: |
The Boeing Company (Seattle,
WA)
|
Family
ID: |
23666280 |
Appl.
No.: |
09/420,391 |
Filed: |
October 19, 1999 |
Current U.S.
Class: |
359/279;
356/5.09; 359/245; 359/340; 372/6 |
Current CPC
Class: |
G02F
1/0121 (20130101); G02F 2203/18 (20130101); G02F
2203/50 (20130101) |
Current International
Class: |
G02F
1/01 (20060101); G02F 001/01 () |
Field of
Search: |
;359/279,245,340,341
;372/6 ;356/5.09 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Epps; Georgia
Assistant Examiner: Thompson; Tim
Attorney, Agent or Firm: Alston & Bird LLP
Claims
What is claimed is:
1. An apparatus for compensating for phase shifts in an optical
power signal, wherein said apparatus comprises:
an optical phase modulator for altering the phase of the optical
power signal;
a detector assembly that receives an interference signal generated
by optical interference of the optical power signal and a reference
signal, wherein said detector assembly generates a lockin signal
that is representative of an optical phase difference between the
optical power signal and the reference signal; and
a processor in electrical communication with both said detector
assembly and said optical phase modulator, wherein said processor
receives the lockin signal and compares the lockin signal to at
least one predetermined set point value, wherein if the lockin
signal exceeds the set point value, said processor alters the
lockin signal by combining the lockin signal with a step waveform
to thereby produce a combined feedback signal that has a value
within desired operation parameters of said optical phase
modulator.
2. An apparatus according to claim 1, wherein said processor
compares the lockin signal to two predetermined set point values
defining a range of lockin values that are within desired operation
parameters of said optical phase modulator, and wherein if the
value of the lockin signal is outside the range of lockin values
defined by the set point values, said processor combines the lockin
signal with a step waveform.
3. An apparatus according to claim 2, wherein if the lockin signal
is outside the range of feedback values defined by the set point
values, said processor combines the lockin signal with a step
exponential waveform, wherein the step exponential waveform has an
initial step that is 2.pi.n, where n is an integer.
4. An apparatus according to claim 3, wherein if the lockin signal
is outside the range of lockin values defined by the set point
values, said processor combines an exponential step function with
the lockin signal to generate a combined feedback signal that is
altered by a factor of 2.pi.n, where n is an integer.
5. An apparatus according to claim 1, wherein the set point value
defines a maximum desired lockin signal, and wherein if the lockin
signal exceeds the set point value, said processor combines the
lockin signal with a step waveform.
6. An apparatus according to claim 5, wherein if the lockin signal
exceeds the set point value, said processor combines the lockin
signal with a step exponential waveform, wherein the step
exponential waveform has an initial step that is 2.pi.n, where n is
an integer.
7. An apparatus according to claim 5, wherein if the feedback
signal exceeds the set point value, said processor combines an
exponential step function with the lockin signal to generate a
combined feedback signal that is altered by a factor of 2.pi.n,
where n is an integer.
8. An apparatus according to claim 1, wherein said detector
assembly comprises a detector that receives the interference signal
and an lockin amplifier in electrical communication with said
detector, wherein said lockin amplifier generates the lockin signal
that is representative of the optical phase difference between the
reference and optical power signals.
9. An apparatus according to claim 8 further comprising a reference
phase modulator optically upstream from said detector and a
frequency generator in electrical communication with said reference
phase modulator, wherein said frequency generator generates a
selected frequency signal, and wherein said reference phase
modulator receives the selected frequency signal and modulates the
reference signal at the selected frequency prior to the
interference of the reference and optical power signals.
10. An apparatus according to claim 9, wherein said lock-in
amplifier is in electrical communication with both said detector
and said frequency generator and receives both the selected
frequency signal and the interference signal, and wherein said
lock-in amplifier generates a feedback signal representing a
component of an optical phase difference between the optical signal
and the reference signal at the frequency of the selected frequency
signal.
11. An apparatus according to claim 1 further comprising at least
one fiber amplifier optically upstream from said detector assembly,
wherein said fiber amplifier amplifies at least one of the
reference and optical power signals.
12. An apparatus according to claim 1 further comprising a gain
amplifier in electrical communication with both said processor and
said optical phase modulator, wherein said gain amplifier amplifies
the combined feedback signal generated by said processor such that
the combined feedback signal has sufficient gain for application to
said optical phase modulator.
13. A method for compensating for phase shifts in an optical power
signal generated by an optical device having an optical phase
modulator for altering the phase of the optical power signal,
wherein said method comprises the steps of:
receiving an interference signal generated by optical interference
of an optical signal and a reference signal;
generating a lockin signal representative of an optical phase
difference between the optical power signal and the reference
signal;
comparing the lockin signal to at least one predetermined set point
value; and
combining the lockin signal with a step waveform to create a
combined feedback signal if the lockin signal exceeds the set point
value, such that the combined feedback signal has a value within
desired operation parameters of said optical phase modulator.
14. A method according to claim 13, wherein said comparing step
comprises comparing the lockin signal to two predetermined set
point values defining a range of lockin values that are within
desired operation parameters of the optical phase modulator, and
wherein if the value of the lockin signal is outside the range of
lockin values defined by the set point values, said combining step
comprises combining the lockin signal with a step waveform.
15. A method according to claim 14, wherein if the value of the
lockin signal is outside the range of lockin values defined by the
set point values, said combining step comprises combining the
lockin signal with a step exponential waveform, wherein the step
exponential waveform has an initial step that is 2.pi.n, where n is
an integer.
16. A method according to claim 13, wherein the set point value
defines a maximum lockin signal, wherein if the lockin signal
exceeds the set point value, said combining step comprises
combining the lockin signal with a step waveform.
17. A method according to claim 16, wherein if the lockin signal
exceeds the set point value, said combining step comprises
combining the lockin signal with a step exponential waveform,
wherein the step exponential waveform has an initial step that is
2.pi.n, where n is an integer.
18. A method according to claim 16, wherein if the lockin signal
exceeds the set point value, said combining step comprises the step
of combining an exponential step function with the lockin signal to
generate a combined feedback signal that is altered by a factor of
2.pi.n, where n is an integer.
19. A method according to claim 13 further comprising prior to said
detecting step the steps of generating a selected frequency signal
and modulating the reference signal at the selected frequency prior
to the interference of the reference and optical power signals.
20. A method according to claim 19, wherein said generating step
comprises generating a lockin signal representative of a component
of an optical phase difference between the optical power signal and
the reference signal at the frequency of the selected frequency
signal.
21. A method according to claim 13 further comprising the step of
amplifying at least one of the reference and optical power signals
prior to said detecting step.
22. A method according to claim 13 further comprising the step of
gain amplifying the feedback signal generated in said generating
step such that the feedback signal has sufficient gain for
application to the optical phase modulator.
23. An apparatus for individually compensating for phase shifts in
each of a plurality of optical power signals, wherein said
apparatus comprises:
a plurality of optical phase modulators for altering the phase of
an associated optical power signal;
a plurality of detector assemblies that each receive an
interference signal generated by optical interference of an optical
power signal and a reference signal associated with each detector
assembly, wherein each of said detector assemblies generates a
respective lockin signal representative of an optical phase
difference between the optical power signal and the reference
signal associated with said detector assembly; and
a processor in electrical communication with both said plurality of
detector assemblies and said plurality of optical phase modulators,
wherein said processor receives a lockin signal generated by each
of said detector assemblies and compares the respective lockin
signal to at least one predetermined set point value for the
respective optical phase modulator, wherein if the lockin signal
exceeds the set point value said processor alters the lockin signal
by combining the lockin signal with a step waveform to thereby
produce a combined feedback signal that has a value within desired
operation parameters of said optical phase modulator.
24. An apparatus according to claim 23, wherein said processor
compares the respective lockin signal to two predetermined set
point values defining a range of lockin values that are within
desired operation parameters of said respective optical phase
modulator, and wherein if the lockin signal is outside the range of
lockin values defined by the set point values, said processor
combines the lockin signal with a step waveform.
25. An apparatus according to claim 24, wherein if the lockin
signal is outside the range of lockin values defined by the set
point values, said processor combines the lockin signal with a step
exponential waveform, wherein the step exponential waveform has an
initial step that is 2.pi.n, where n is an integer.
26. An apparatus according to claim 24, wherein if the lockin
signal is outside the range of lockin values defined by the set
point values, said processor, said processor combines an
exponential step function with the lockin signal to generate a
combined feedback signal that is altered by a factor of 2.pi.n,
where n is an integer.
27. An apparatus according to claim 23, wherein the set point value
defines a maximum lockin signal value, and wherein if the lockin
signal exceeds the set point value, said processor combines the
lockin signal with a step waveform.
28. An apparatus according to claim 27, wherein if the respective
lockin signal exceeds the set point value, said processor combines
the lockin signal with a step exponential waveform, wherein the
step exponential waveform has an initial step that is 2.pi.n, where
n is an integer.
29. An apparatus according to claim 28, wherein if the respective
lockin signal exceeds the set point value, said processor combines
an exponential step function with the lockin signal to generate a
combined feedback signal that is altered by a factor of 2.pi.n,
where n is an integer.
30. An apparatus according to claim 23, wherein each of said
plurality of detector assemblies comprises a detector that receives
a respective interference signal and a lockin amplifier in
electrical communication with said detector, wherein said lockin
amplifier generates the lockin signal that is representative of the
optical phase difference between the respective reference and
optical power signals.
31. An apparatus according to claim 30 further comprising a
plurality of reference phase modulators optically upstream from
each of said plurality of detectors and a frequency generator in
electrical communication with each of said reference phase
modulators, wherein said frequency generator generates a selected
frequency signal, and wherein each of said reference phase
modulators receive the selected frequency signal and modulate a
respective reference signal at the selected frequency prior to the
interference of the reference and optical power signals.
32. An apparatus according to claim 31, wherein each of said
lock-in amplifiers is in electrical communication with both a
respective detector and said frequency generator and receives both
the selected frequency signal from said frequency generator and an
interference signal from said detector, and wherein each of said
lock-in amplifiers generates a lockin signal representative of a
component of an optical phase difference between the optical power
signal and the reference signal at the frequency of the selected
frequency signal.
33. An apparatus according to claim 23 further comprising a
plurality of fiber amplifiers optically upstream from said
plurality of detector assemblies, wherein said fiber amplifiers
amplify at least one of an associated reference and optical power
signals.
34. An apparatus according to claim 23 further comprising a
plurality of gain amplifiers in electrical communication with both
said processor and a respective optical phase modulator, wherein
said gain amplifiers amplify an associated combined feedback signal
generated by said processor such that the associated combined
feedback signal has sufficient gain for application to said optical
phase modulator.
35. An apparatus according to claim 23 further comprising a
collimating lens upstream from said plurality of detector
assemblies, wherein said collimating lens collimates the reference
signal.
36. A computer program product for compensating for phase shifts in
an optical power signal generated by an optical device, wherein the
optical device includes an optical phase modulator for altering the
phase of the optical signal and a detector assembly that generates
a lockin signal that is representative of an optical phase
difference between the optical power signal and a reference signal,
wherein the computer program product comprises:
a computer readable storage medium having computer readable program
code means embodied in said medium, said computer-readable program
code means comprising:
first computer-readable program code means for comparing the lockin
signal to at least one predetermined set point value; and
second computer-readable program code means for combining the
lockin signal with a step waveform to create a combined feedback
signal if the lockin signal exceeds the set point value, such that
the combined feedback signal has a value within desired operation
parameters of said optical phase modulator.
37. A computer program product as defined in claim 36, wherein said
first computer-readable program code means comprises computer
readable program code means for comparing the lockin signal to two
predetermined set point values defining a range of feedback values
that are within desired operation parameters of the optical phase
modulator, and wherein said second computer-readable program code
means comprises computer readable program code means for combining
the lockin signal with a step waveform, if the value of the lockin
signal is outside the range of lockin values defined by the set
point values.
38. A computer program product as defined in claim 37, wherein said
second computer-readable program code means comprises computer
readable program code means for combining the lockin signal with a
step exponential waveform, wherein the step exponential waveform
has an initial step that is 2.pi.n, where n is an integer, if the
value of the lockin signal is outside the range of lockin values
defined by the set point values.
39. A computer program product as defined in claim 37, wherein said
second computer-readable program code means comprises computer
readable program code means for combining an exponential step
function with the lockin signal to create a combined feedback
signal that is altered by a factor of 2.pi.n, where n is an
integer, if the value of the lockin signal is outside the range of
lockin values defined by the set point values.
40. A computer program product as defined in claim 36, wherein the
set point value defines a desired maximum lockin signal, and
wherein said second computer-readable program code means comprises
computer readable program code means for combining the lockin
signal with a step waveform, if the lockin signal exceeds the set
point value.
41. A computer program product as defined in claim 40, wherein said
second computer-readable program code means comprises computer
readable program code means for combining the lockin signal with a
step exponential waveform, wherein the step exponential waveform
has an initial step that is 2.pi.n, where n is an integer, if the
lockin signal exceeds the set point value.
42. A computer program product as defined in claim 40, wherein said
second computer-readable program code means comprises computer
readable program code means for combining an exponential step
function with the lockin signal to create a combined feedback
signal that is altered by a factor of 2.pi.n, where n is an
integer, if the lockin signal exceeds the set point value.
Description
FIELD OF THE INVENTION
The present invention relates generally to laser systems and more
particularly to control systems for controlling the phase of
optical signals output by laser systems.
BACKGROUND OF THE INVENTION
Lasers are presently employed for a wide variety of applications.
For example, lasers are employed to process materials, such as by
cutting, welding, heat treating, drilling, trimming and coating
materials, stripping paint, removing coatings, cleaning surfaces,
and providing laser markings. Lasers are also used in many medical
applications for precision surgery. Additionally, lasers are used
in military applications, including laser weapon and laser ranging
systems. Laser communication systems have also been developed in
which laser signals are transmitted in a predetermined format to
transmit data.
Along with the ever increasing number of applications in which
lasers are used, the demands on the laser systems are also ever
increasing. For example, a number of applications, including
military, materials processing, medical, and communications
applications, demand continuous wave lasers which emit increasingly
higher power levels. In addition, a number of applications demand
that the laser system produce an output beam which is of high
quality, e.g., exhibiting predominantly or entirely fundamental or
TEM.sub.00 mode characteristics. Accordingly, the output beam can
be more definitely focused to achieve higher brightness. At the
same time, many applications require that the laser system produce
an output beam which is adaptable or dynamically controllable.
One example of the need for high power, high quality laser beams is
illustrated in laser devices used for focusing on remote targets.
In these applications, it advantageous for the laser beam to
achieve a maximum brightness at the location of the target. For
example, in military applications, it is advantageous to generate a
laser beam that is focused on the remote target with maximum
intensity. Similarly, in medical applications, it is essential that
the laser beam is focused at the target tissue such that
surrounding tissue is not affected. However, an overall problem
with the control of laser beams is perturbations in the atmosphere
in which the laser beam propagates. These perturbations degrade the
laser beam, deflect the laser beam, and reduce laser power.
To address the problems associated with these perturbations and
provide control of the laser beams, devices have been developed
that sense the perturbations occurring in the path of the laser
beam and compensate for these perturbations by adjusting the laser
beam. For example, The Boeing Company, assignee of the present
application, has developed several different types of laser devices
that generate high powered, turbulence compensated laser beams.
Examples of these device are discussed in detail in U.S. Pat. No.
5,694,408 to Bott et al., U.S. Pat. No. 5,847,816 to Zediker et
al., and U.S. Pat. No. 5,832,006 to Rice et al., the contents of
which are incorporated herein by reference.
The basic approach to many of these devices is to amplify a
coherent signal emitted from a master oscillator using a phased
array of fiber optic amplifiers. A portion of the output optical
signal referred to as a power signal is extracted for comparison to
a reference laser beam also output by the master oscillator. The
power signal and the reference signal are combined by interference,
and the interference signal is sampled by an array of detectors.
The difference in phase between the power and reference signal is
recorded by the detector, and is used as feedback for altering the
phase modulation of the power signal via an array of phase
modulators that are in optical communication with respective fiber
optic amplifiers.
As an example, in one application, a reference beam is initially
transmitted to a target of interest, and the reflection of the beam
indicates atmospheric turbulence in the path of the output laser
beam. To counteract these turbulence, the device alters the phase
of signals emitted by the various fiber optic amplifiers such that
the output laser has a wavefront that compensates for the
atmospheric turbulence. An important component of this device is
the feedback loop used to control the phase modulation of the
output laser beam. Specifically, as discussed, a portion of the
output laser beam is combined through interference with a reference
signal to determine the phase difference for the signals emitted by
each fiber optic amplifier. By use of the feedback signal
representative of the phase of the output laser beam and knowledge
of the desired wavefront, the output laser beam can be controlled
via the array of phase modulators to produce the desired wavefront
and/or to appropriately steer or tilt the wavefront.
An important aspect of these laser devices is the control of the
phase of the output laser beam by the array of phase modulators. As
discussed, these phase modulators are controlled by a feedback
signal representing the difference in phase of a portion of the
output laser beam called the power signal and the reference signal.
Although these systems, for the most part, provide reliable and
accurate control of the output laser beam, problems may be
encountered when the feedback signal exceeds a desired maximum
feedback value such that saturation may occur. An additional
problem is experienced when the feedback signal causes uncontrolled
modulation changes in the output signal.
The problems associated with the phase modulators are illustrated
with reference to FIG. 1. FIG. 1 is a block diagram representation
of a typical Mach-Zehnder interferometer that uses a feedback
signal to correct for phase differences. Specifically, the laser
device 10 includes master oscillator 12 that produces an output
optical signal 14. This output optical signal is directed to a beam
splitter 16 that creates a power and a reference signal 18. The
reference and power signals are amplified by respective fiber
amplifiers 20 and 22 and pump sources 24 and 26. The reference and
power signals are collimated by collimating lenses 28 and 30. A
portion of the power signal is separated by a beam splitter 32, and
the reference signal and the power signal are combined by
interference to produce an interference signal 34. The interference
signal is detected by a detector 36 and supplied to a lockin
amplifier 38.
Prior to interference with the output signal, the reference signal
is modulated with a reference phase modulator 40 at a predetermined
frequency provided by a frequency generator 42. The predetermined
frequency is also supplied to the lockin amplifier 38, which
generates a lockin signal that is proportional to the sine of the
phase difference between the reference and power signals at the
predetermined frequency. This lockin signal is provided to an
optical phase modulator 44, which alters the phase of the power
signal to substantially match the phase of the reference signal. As
such, the phase of the output signal may be regulated. The feedback
loop may also include a gain amplifier 46 that amplifies the lockin
signal prior to input into the phase modulator.
As discussed above, problems may be encountered where the lockin
signal either exceeds the maximum input of the phase modulator or
the lockin signal is such that it may introduce uncontrollable
phase changes in the output signal. This problem is more
specifically illustrated with reference to FIGS. 2 and 3. FIG. 2
illustrates in block diagram form the feedback loop of the laser
device of FIG. 1. Specifically, the interference signal received by
the detector represents phase difference .DELTA..phi..sub.tot
between the reference and output signals and consists of gain
differences and environmental phase differences
.DELTA..phi..sub.Env. The interference signal 34, shown in FIG. 1,
is provided to the lockin amplifier, which, in turn, generates a
lockin signal that is proportional to the sine of the phase
difference between the reference and output signals (i.e.,
sin(.DELTA..phi..sub.tot)). The lockin signal is also gain
amplified by the gain amplifier 46 (i.e.,
gsin(.DELTA..phi..sub.tot)), prior to being presented to the phase
modulator, which alters, i.e., reduces, the phase of the signal by
a corresponding amount. As such, the relationship between
.DELTA..phi..sub.tot and .DELTA..phi..sub.Env is expressed as
follows:
As illustrated by the equation, a shift in environmental phase
between the reference and power signal will directly affect the
value of the feedback signal. With reference to FIG. 3, the drift
in phase between the reference and output signals of a typical
optic interferometer is illustrated. As can be seen, the
interferometer may experience large phase drifts due to
environmental causes over a short period of time. These large phase
drifts may cause problems with operation of the phase modulators.
Specifically, typical phase modulators have maximum input limits,
above which, the phase modulators will saturate. In systems such as
described above, if the environmental changes generate a signal
that is greater than a desired maximum lockin signal, saturation
may occur, which will affect operation of the laser device.
An additional concern with the feedback signal is related to
sinusoidal aspects of the feedback signal. With reference to FIG.
4, the feedback signal for a particular gain value of g=10 is
plotted in terms of .DELTA..phi..sub.tot vs. .DELTA..phi..sub.Env.
As can be seen from this graphic representation, for certain
environmental phase difference values, the change in total phase
may be such that modulation of the power signal is uncontrollable.
For example, if the environmental phase difference is currently
.DELTA..phi..sub.Env /2.pi..apprxeq.0.56, then the total phase
difference is .DELTA..phi..sub.tot /2.pi..apprxeq.-0.8. However, if
in the next instant, the environmental phase changes to
.DELTA..phi..sub.Env /2.pi..apprxeq.0.77, the total phase
difference will change to .DELTA..phi..sub.tot /2.pi..apprxeq.-0.5.
This change in phase may cause uncontrollable phase changes in the
output signal.
SUMMARY OF THE INVENTION
As set forth below, the present invention provides several
apparatus, methods, and computer program products that may overcome
many of deficiencies detailed above concerning the phase control of
laser devices. The present invention provides various apparatus,
methods, and computer program products that analyze the lockin
signal in a feedback loop of an optical interferometer prior to
application of the lockin signal to the optical phase modulator.
Specifically, the present invention compares the lockin signal to
at least one set point value, and if the lockin signal exceeds the
set point value, the present invention combines the lockin signal
with a step waveform to produce a combined feedback signal that has
a value within desired operation parameters of the optical phase
modulator. By initially analyzing the lockin signal prior to input
into the optical phase modulator, the present invention can prevent
saturation of the laser system and also alleviate uncontrollable
changes in the modulation of the power signal.
For example, in one embodiment of the present invention, the
predetermined set point value represents a desired maximum lockin
signal value above which saturation may occur. In this embodiment,
the present invention analyzes the lockin prior to application to
the optical phase modulator to determine whether the lockin exceeds
the predetermined set point value. If the lockin signal exceeds the
predetermined set point value, the present invention alters the
lockin signal prior to application to the optical phase modulator,
such that saturation does not occur.
In another embodiment of the present invention, the predetermined
set point is actually two predetermined set point values that
define a range of lockin values that are within desired operation
parameters of the optical phase modulator. Specifically, in this
embodiment of the present invention, lockin values that are not
within the range of the two set point values may cause
uncontrollable changes in modulation of the output signal. In this
embodiment, the present invention analyzes the lockin signal prior
to its application to the optical phase modulator to determine
whether the lockin signal is outside the range of the two set point
values. If the lockin signal is outside the range of the set point
values, the present invention alters the lockin signal prior to
application to the optical phase modulator, such that there are not
uncontrollable phase changes in the output signal of the laser
system. Additionally, the present invention may be implemented to
control the phase modulation of a single output signal or it may be
implemented in a system to control a plurality of output
signals.
These and other advantages are provided, according to the present
invention, by an apparatus for compensating for phase shifts in an
optical signal generated by an optical device. The apparatus of
this embodiment includes an optical phase modulator for altering
the phase of the optical signal. Additionally, the apparatus of
this embodiment includes a detector assembly for receiving an
interference signal generated by optical interference of a power
signal and a reference signal and generating a lockin signal
representative of a phase difference between the power and
reference signals. Optically connected to both the optical phase
modulator and the detector assembly is a processor for analyzing
the lockin signal.
In operation, the detector initially receives an interference
signal representing the interference of a reference and power
signal. The detector assembly generates a lockin signal
representing the phase difference between the reference and power
signals. The processor receives the lockin signal and compares the
lockin signal to at least one predetermined set point value. If the
lockin signal exceeds the set point value, the processor combines
the lockin signal with a step waveform to generate a combined
feedback signal having a value within desired operation parameters
of the optical phase modulator.
As discussed above, the apparatus of the present invention compares
the lockin signal to the predetermined set point value. In one
embodiment of the present invention, the set point value defines a
desired maximum lockin value above which saturation may occur. In
this embodiment of the present invention, the processor compares
the lockin signal to the predetermined set point value, and if the
lockin signal exceeds the set point value, the processor combines
the lockin signal with a step waveform to generate a combined
feedback signal having a value within desired operation parameters
of the optical phase modulator.
In another embodiment of the present invention, the predetermined
set point value is two predetermined set point values defining a
range of feedback values that are within desired operation
parameters of the optical phase modulator. Specifically, in this
embodiment, feedback signals having values outside the range of the
set point values may cause uncontrollable modulation changes. As
such, in this embodiment, if the value of the lockin signal is
outside the range of lockin values between the set point values,
the processor combines the lockin signal with a step waveform to
generate a combined feedback signal having a value within desired
operation parameters of the optical phase modulator.
In some embodiments of the present invention, the detector assembly
generates a feedback signal representing the sine of the phase
difference between the reference and optical signals. In this
embodiment of the present invention, the feedback signal may have a
value that is a multiple of 2.pi. plus some value, i.e., 2.pi.n+x.
For example, the feedback signal may have a value such as 0, 2.pi.,
2.1.pi., 3.3.pi., 4.3.pi., etc. Since sin(x) and sin(2.pi.n+x) is
the same, the processor may combine the lockin signal with a step
waveform having an initial step that is 2.pi.n, where n is an
integer, to create a combined feedback signal that is within the
operating parameters of the optical phase modulator. Because the
lockin signal has been altered by 2.pi.n, the optical phase
modulator will alter the phase of the power signal correctly.
As discussed above, the processor of the present invention combines
the lockin signal with a step waveform if the feedback signal
exceeds the predetermined set point value. The alteration of the
lockin signal may be achieved using several different waveforms.
For example, the processor combines a step exponential function
with the lockin signal to generate a combined feedback signal
having an amplitude that is altered by a factor of 2.pi.n, where n
is an integer.
As discussed above, the apparatus of the present invention includes
a detector assembly for detecting the interference signal and
generating a feedback signal representing the phase difference
between the reference and output signals. In one embodiment of the
present invention, the detector assembly includes a detector for
detecting the interference signal and a lockin amplifier for
generating the feedback signal. In this embodiment of the present
invention, the apparatus also includes a reference phase modulator
optically upstream from the detector and a frequency generator
connected to both the reference phase modulator and the lockin
amplifier.
In this embodiment of the present invention, the frequency
generator supplies a reference frequency to both the lockin
amplifier and the reference phase modulator. The reference phase
modulator is connected to the reference signal and dithers the
reference signal at the reference frequency supplied by the
frequency generator prior to the reference signal interfering with
the power signal. The detector detects the interference signal
created by the interference of the reference and power signals. The
lockin amplifier receives the interference signal and generates a
lockin signal representing the sine of an optical phase difference
between the power signal and the reference signal. Finally, the
processor analyzes the lockin signal to determine whether the
lockin signal should be altered prior to introduction to the
optical phase modulator.
In addition to providing apparatus and methods for analyzing the
lockin signal such that the lockin signal does not exceed
predetermined set points, the present invention also provides
computer program products. The computer program products of the
present invention are similar to the apparatus and methods of the
present invention in that they compensate for phase shifts in an
optical signal generated by an optical device having an optical
phase modulator that alters the phase of the optical signal and a
detector assembly that generates a lockin signal representing an
optical phase difference between a power signal and a reference
signal.
The computer program products of the present invention include a
computer readable storage medium having computer readable program
code means embodied in the medium. The computer-readable program
code means includes first computer-readable program code means for
comparing the lockin signal to at least one predetermined set point
value, and second computer-readable program code means for
combining the lockin signal with a step waveform to create a
combined feedback signal, if the lockin signal exceeds the set
point value, such that the combined feedback signal has a value
within desired operation parameters of the optical phase modulator.
As such, the computer program product of the present invention can
also prevent saturation and alleviate uncontrollable phase changes
in the modulation of the output signal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an optical interferometer in which the
apparatus of the present invention may be implemented.
FIG. 2 is a block diagram of the feedback control loop for the
optical interferometer of FIG. 1.
FIG. 3 is a graphic representation of the difference in phase
between the reference and output signals of the optical
interferometer of FIG. 1 over a period of time.
FIG. 4 is a graphic representation of the feedback signal
illustrated in FIG. 2.
FIG. 5 is a block diagram of an apparatus for controlling the phase
of an output signal in a laser system according to one embodiment
of the present invention.
FIG. 6 is a block diagram of the operations performed to control
the phase of an output signal in a laser system according to one
embodiment of the present invention.
FIG. 7 is a graphic representation of the feedback signal
illustrated in FIG. 2 along with the plot of limit lines used to
determine the set point values according to one embodiment of the
present invention.
FIGS. 8A-8C are graphic representations of the feedback signal,
scale factor function, and lockin amplifier signal according to one
embodiment of the present invention.
FIGS. 9A-9B are graphic representations of the feedback signal and
output signal for the implementation of the apparatus of the
present invention in a laser system.
FIG. 10 is a block diagram of an apparatus for controlling the
phase of an output signal in a high energy laser system that
includes a near field modulation technique according to one
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention now will be described more fully hereinafter
with reference to the accompanying drawings, in which preferred
embodiments of the invention are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout.
As discussed above, the present invention provides various
apparatus, methods, and computer program products for compensating
for phase shifts in an optical signal generated by an optical
device. Specifically, the present invention provides various
apparatus, methods, and computer program products that analyze the
lockin signal of an optical interferometer prior to application of
the lockin signal to an optical phase modulator. The present
invention compares the lockin signal to at least one set point
value, and if the lockin signal exceeds the set point value, the
present invention combines the lockin signal with a step waveform
to generate a combined feedback signal having a value within
desired operation parameters of the optical phase modulator. By
initially analyzing the lockin signal prior to input into the
optical phase modulator, the present invention can prevent
saturation and also alleviate uncontrollable changes in the
modulation of the output signal.
With relation to the description of the various embodiments of the
present invention provided in detail below, it must be understood
that the present invention can be used with laser systems that use
feedback loops to control the phase of an optical signal output by
the laser system. However, the various apparatus, methods, and
computer program products of the present invention have been
illustrated below with reference to two specific laser systems for
illustrative purposes. As this disclosure is for illustrative
purposes only, the scope of the present invention should not limit
the use of the present invention with other systems, as the
concepts and designs described below may be implemented in any
laser system that uses a feedback loop to control the phase of an
optical signal output by the laser system.
With reference to FIG. 5, an apparatus for compensating for phase
shifts in an optical signal generated by an optical device
according to one embodiment of the present invention is illustrated
in conjunction with a Mach-Zehnder interferometer as illustrated in
FIG. 1. As previously detailed in FIG. 1, the interferometer 10
includes a master oscillator 12 for producing an output optical
signal 14. Optically connected to the master oscillator is a beam
splitter 16 for separating the output signal into a reference
signal and a power signal. Optically connected to the outputs of
the beam splitter are respective fiber amplifiers 20 and 22 and
pump sources 24 and 26, which amplify the power and reference
signals. Downstream from the fiber amplifiers are collimating
lenses 28 and 30 and a beam splitter 32 for collimating the power
and reference signals and combining the reference and power signals
by interference. Upstream from the collimating lens 28 that
collimates the reference signal, the interferometer includes a
reference phase modulator 40 in optical communication with the
reference signal and a frequency generator 42 for dithering the
reference signal at a predetermined frequency.
One embodiment of the apparatus of the present invention also
includes an optical phase modulator 46 in optical communication
with the output signal for controlling the phase of the optical
signal. The apparatus of the present invention further includes a
detector assembly 48 for receiving the interference signal
generated by the optical interference of the power signal and the
reference signal and generating a lockin signal. Connected to both
the detector assembly and the optical phase modulator is a
processor 50 for analyzing the lockin signal prior to application
of the lockin signal to the optical phase modulator.
In operation, the master oscillator initially generates an output
signal and the beam splitter splits the optical signal into a power
and a reference signal. The reference and power signals are
typically amplified by respective fiber amplifiers and pump
sources. The reference signal is next dithered by the reference
phase modulator at a frequency set by the frequency generator. The
dithered reference signal and the power signal are next collimated
by the collimating lenses. The reference signal and power signals
are then combined by interference to produce an interference
signal.
With reference to FIG. 6, the operation of the apparatus of the
present invention to control the phase modulation of the laser
system is shown. In this embodiment of the present invention, the
detector assembly receives the interference signal produced by the
combination of the reference and power signals. (See step 200). The
detector assembly generates a lockin signal representing the
difference in phase between the reference and output signals. (See
step 210). The processor receives the lockin signal and compares
the lockin signal to a predetermined set point value. (See step
220). If the lockin signal exceeds the predetermined set point
value, the processor combines the lockin signal with a step
waveform to generate a combined feedback having a value within
desired operation parameters of the optical phase modulator. (See
step 230). If the lockin signal does not exceed the predetermined
set point value, the lockin signal is not altered. The feedback
signal or the lockin signal, if it is not altered, is next provided
to the optical phase modulator, where the feedback signal or
unaltered lockin signal is used to alter the phase of the output
signal. (See step 240).
As discussed above, the present invention includes a detector
assembly for receiving an interference signal and generating a
feedback signal representative of the interference signal. With
reference to FIG. 5, in one embodiment of the present invention,
the detector assembly 48 includes a detector 52 and a lockin
amplifier 54. In this embodiment of the present invention, the
detector receives the interference signal generated by the
interference of the reference and power signals. The lockin
amplifier, in turn, generates a lockin signal that is proportional
to the sine of the phase difference between the reference and power
signals. The lockin amplifier of this embodiment is a commercially
available device manufactured by Stanford Research Systems located
in Sunnyvale, Calif.
As discussed, the present invention provides apparatus, methods,
and computer program products that analyze the lockin signal prior
to application to the optical phase modulator such that the lockin
signal is within the operating parameters of the optical phase
modulator. Importantly, the present invention alters the lockin
signal such that the lockin signal is at an appropriate value for
application to the optical phase modulator. For instance, in one
embodiment of the present invention, there may be a maximum desired
input value from the feedback value, above which saturation may
occur. In this embodiment of the present invention, the
predetermined set point value may represent a maximum desired
feedback value.
With reference to FIG. 6, in this embodiment, the processor
receives the lockin signal and compares the lockin signal to the
predetermined set point value. (See step 220). If the lockin signal
exceeds the predetermined set point value, the processor combines
the lockin signal with a step waveform and generates a combined
feedback signal having a value within desired operation parameters
of the optical phase modulator. (See step 230). The combined
feedback signal or lockin signal, if not altered, is next provided
to the optical phase modulator, where the feedback or lockin signal
is used to alter the phase of the output signal, with reduced risk
of saturation. (See step 240).
As discussed earlier with reference to FIG. 4, in embodiments in
which a lockin amplifier is used, the lockin amplifier generates a
lockin signal that represents the sine of the phase difference
between the reference and power signals. As shown in FIG. 4, in
this embodiment, due to the sinusoidal nature of the lockin signal
generated by the lockin amplifier (i.e.,
gsin(.DELTA..phi..sub.tot)), some values of the lockin signal may
cause uncontrollable modulation of the output signal. For this
reason, in one embodiment of the present invention, the
predetermined set point value may represent a range of lockin
values, where lockin values outside this range may cause the
optical phase modulator to experience uncontrollable
modulation.
With reference to FIG. 6, in this embodiment of the present
invention, the lockin amplifier of the detector assembly generates
a lockin signal and the processor receives the lockin signal. The
processor compares the lockin signal to the predetermined set point
values. (See step 220). If the lockin signal is not within the
range of lockin signals defined by the predetermined set point
values, the processor combines the lockin signal with a step
waveform and generates a combined feedback signal having a value
within desired operation parameters of the optical phase modulator.
(See step 230). The feedback signal or the lockin signal, if not
altered, is next provided to the optical phase modulator, where the
feedback of lockin signal is used to alter the phase of the output
signal, with reduced risk of uncontrolled modulation. (See step
240).
As detailed above, the processor of this embodiment compares the
lockin signal to predetermined set point values defining a range of
acceptable lockin values. With reference to FIG. 7, the
determination of the set point values used by the processor in this
embodiment of the present invention is illustrated. FIG. 7 is a
plot of the feedback signal with a gain value of g=10 as previously
illustrated in FIG. 4. Specifically, FIG. 7 is a plot in terms of
the total phase difference .DELTA..phi..sub.tot between the
reference and power signals and the phase difference between the
reference and power signals due to environmental differences
.DELTA..phi..sub.Env and is defined by the equation:
As discussed with relation to FIG. 4, due to the sinusoidal nature
of the lockin signal generated by the lockin amplifier, certain
values of the lockin signal may cause uncontrollable modulation
changes. For example, if the environmental phase difference is
currently .DELTA..phi..sub.Env /2.pi..apprxeq.0.56, then the total
phase difference is .DELTA..phi..sub.tot /2.pi..apprxeq.-0.8.
However, if in the next instant, the environmental phase changes to
.DELTA..phi..sub.Env /2.pi..apprxeq.0.77, the total phase
difference will change to .DELTA..phi..sub.tot /2.pi..apprxeq.-0.5.
This change in phase may cause uncontrollable modulation changes.
To remedy these problems, the predetermined set points should be
chosen such that lockin values that may cause uncontrolled
modulation are altered prior to application to the optical phase
modulator.
With reference to FIG. 7, to define the predetermined set point
values, the equation for the feedback signal is plotted in terms of
total phase difference of the reference and power signal
.DELTA..phi..sub.tot and the change in phase due to environmental
effects .DELTA..phi..sub.Env. Limit lines 56 and 58 are then drawn
such that the regions between the limit lines define lockin values
that will not cause uncontrollable modulation changes. The limit
lines are drawn for several 2.pi.n factors. These limit lines
define the set point values that are used by the processor to
compare to the lockin signal, such that lockin signal values that
are not within the range of the set points should be altered prior
to application to the optical phase modulator. These determined set
point values are typically stored in the processor and used to
evaluate the lockin signal.
As discussed above, the present invention combines the lockin
signal with a step waveform if the value of the lockin signal is
not within the operation parameters of the optical phase modulator.
Although the step waveform may be any type of waveform, in one
embodiment, the step waveform is a step exponential waveform, where
the step exponential waveform has an initial step that is 2.pi.n,
where n is an integer that then exponentially returns to the
original level. In this embodiment, the equation for the
exponential function is defined as: ##EQU1##
where u(t)=unt step:
t.sub.0 =time of step:
.upsilon.=time constant
The exponential step function also typically has a time constant
(i.e., .tau.) which is preferably equal to the time constant of the
lockin amplifier.
As discussed previously, in some embodiments of the present
invention, the detector assembly includes a lockin amplifier that
generates a feedback signal that is the sine of the phase
difference between the reference and power signals (i.e.,
sin(.DELTA..phi..sub.tot)). Because the lockin signal is
represented in terms of the sine of the phase difference, the
lockin value may be altered either positively or negatively by a
value of 2.pi.n, where n is an integer, without affecting the phase
with which the signal will be modulated.
For example, with reference to FIG. 6, in embodiments where the
lockin signal exceeds the predetermined set point value, the
processor combines the lockin signal with an exponential step
function and generates a combined feedback signal that is altered
by a scale factor of 2.pi.n. (See step 230). As the lockin signal
is the sine of the difference in phase, the processor may alter the
lockin signal by a factor of 2.pi.n bringing the combined feedback
signal into the operation range of the optical phase modulator
without affecting the relative phase at which the optical phase
modulator will modulate the output signal.
The processor may alter the lockin signal by the step waveform in
many ways. For example, the processor may receive the lockin signal
and combine the lockin signal with a step waveform that effectively
subtracts a 2.pi.n to form a combined feedback signal that has a
reduction of 2.pi.n. In some embodiments, however, it may be
preferable to combine the lockin signal with a step waveform that
effectively adds 2.pi.n to form a combined feedback signal that has
an increase of 2.pi.n.
For example, if the maximum desired input value is 2.pi., then the
predetermined set point value may be set to 2.pi. in this
embodiment. As such, if the lockin signal has a value of 2.1.pi.,
the processor will combine the lockin signal with a step waveform
that effectively subtracts a 2.pi. to form a combined feedback
signal that has a reduction of 2.pi..
In embodiments, in which the predetermined set point represents two
set point values defining a range of lockin values that are
acceptable for the operation of the optical phase modulator, the
processor may either increase or decrease the lockin by 2.pi.n. In
one embodiment, the processor determines whether to increase or
decrease the lockin signal based on the drift trend of the lockin
signal. For example, if the lockin signal has drifted upward over
time, the processor my decrease the lockin signal by combining it
with a step waveform having an initial step portion that is 2.pi.n
to create a combined feedback signal that is decreased by 2.pi.n.
Similarly, if the feedback signal has drifted downward over time,
the processor may combine the lockin signal with a step waveform
having an initial step portion that is 2.pi.n to create a combined
feedback signal that is increased by 2.pi.n.
FIGS. 8A-8C are provided to further illustrate the alteration of
the lockin. FIG. 8A is a graphic representation of the combined
feedback signal output by the processor in terms of magnitude
versus time. FIG. 8B is a graphic representation of a exponential
step function that is combined with the feedback signal by the
processor to reduce the lockin signal by 2.pi.. FIG. 8C is a
graphic representation of the lockin signal output by the lockin
amplifier before and after the processor has created the feedback
signal.
With reference to FIG. 8A, at time T1, the feedback signal received
by the processor has drifted above the predetermined set point
value 60. At this point, the processor combines the feedback signal
with the exponential step function of FIG. 8B at time T1. Due to
the combination of the exponential step function of FIG. 8B with
the feedback signal of FIG. 8A, the feedback signal is reduced to a
level designated 62. When the combined feedback signal at the new
level designated 62 is input into the optical phase modulator, the
optical phase modulator alters the phase of the output signal. With
reference to FIG. 8C, based on the altered output signal, the
lockin amplifier generates a lockin signal 64 that is scaled
down.
With reference to FIGS. 9A and 9B, actual data from the use of an
optical interferometer incorporating the present invention is
illustrated. Specifically, FIG. 9A represents graphically the
feedback signal, and FIG. 9B represents the signal output by the
lockin amplifier that is representative of the interference signal
received by the detector. As can be seen from FIG. 9A, at time T1,
the feedback signal has decreased below a predetermined set point
value, and the processor increases the feedback signal by a scale
factor of 2.pi.. Similarly, at time T2, the feedback signal has
increased above a predetermined set point value, and the processor
decreases the feedback signal by a scale factor of 2.pi.. With
reference to FIG. 9B, the signal output by the lockin amplifier
representing the interference signal is maintained at a minimum
indicating that there is minimal phase difference between the
reference and output signal. The only exceptions occur at times T1
and T2, when the reference and power signals are momentarily out of
phase. In typical embodiments, the duration of the spikes at times
T1 and T2 is inversely related to the bandwidth of the optical
phase modulator.
The above embodiments illustrate the use of the present invention
in relation to a typical optical interferometer. It must be
understood that the present invention can be used with laser
systems that use feedback loops to control the phase of an optical
signal output by the laser system. As previously discussed, the
assignee of the present application has developed several laser
systems that control the output laser beam by use of feedback
signals. At least some of these laser systems are disclosed in
detail in U.S. Pat. No. 5,694,408 to Bott et al., the contents of
which are incorporated herein by reference. It must be understood
that the present invention may be used in these laser systems, as
well as many other types of laser systems.
For illustrative purposes, provided below is an embodiment of the
present invention in conjunction with a high energy laser system
that includes a near field modulation technique, as more fully
described in the Bott '408 patent. With reference to FIG. 10, the
laser system of this embodiment includes a master oscillator 66
that provides an output optical signal. Connected to the master
oscillator is a beam splitter 68 that divides a portion of the
signal to form a power signal 78 and a reference signal 80. The
power signal propagates through a power leg 82, and the reference
signal propagates through a reference leg 84. The power leg 82
contains an array of phase modulators 83 for controlling the phase
of individual portions of the power signal and an array of fiber
amplifiers 86 that amplify the individual portions of the power
signal. Additionally, the power leg 82 includes a plurality of fill
lenses 88 and a beam splitter 90.
The reference leg 84 includes a collimating lens 92 for collimating
the reference signal and an array of optical phase modulators 94
for controllably adjusting the respective phases of each of a
plurality of reference signals. In optical communication with the
array of optical phase modulators 94 of the reference leg and the
beam splitter 90 of the output leg is a beam splitter 96. The beam
splitter 96 combines the portion of the plurality of power signals
divided by the beam splitter 90 and the plurality of reference
signals from the array of optical phase modulators 94 to produce an
interference signal.
The laser system also includes an array of wavefront sensors 98
that sense the wavefront of the output signal. Connected to the
array of wavefront sensors is a wavefront processor 100 that
receives a plurality of signals indicating the configuration of the
wavefront of the output signal. The wavefront processor 100 is
connected to the array of optical phase modulators 94 of the
reference leg and controls the reference signal to impose a desired
wavefront upon the optical signal. Importantly, the laser system of
this embodiment initially transmits a test signal to a remote
target. The reflection of the test signal is used to determine the
manner in which the output optical signal is perturbed in the
atmosphere prior to reaching the target. The reflection of this
test signal is used by the wavefront processor 100 to determine the
proper wavefront for the output signal. Specifically, the wavefront
processor controls the reference signal via the array of optical
phase modulators 94 to impose the desired wavefront upon on the
reference signal. In this regard, the wavefront processor and the
array of optical phase modulators can operate to impose a wavefront
that is the phase conjugate of the reflected test signal so as to
offset the anticipated atmospheric perturbations. The reference
signals are combined with the portion of the power signal diverted
by the beam splitter 68. The difference in phase between the
reference and power signal represents the difference between
desired wavefront and the actual wavefront of the output signal. As
such, the interference of the reference and power signals can be
used to drive the output signal to a desired wavefront.
To use the interference signal to control the wavefront of the
output signal, the laser system of the Bott '408 patent uses a
feedback loop that senses the interference signal and uses a
processor to change the phase of the power signal via the optical
phase modulators 83 of the power leg 78. Although this laser system
of the Bott' 408 patent provides a reliable and accurate system for
controlling the wavefront of an output signal, the laser system may
experience some drawbacks due to the array of optical phase
modulators 83. Specifically, as discussed in the previous
embodiments, problems may occur where the feedback signal supplied
to the array of optical phase modulators 83 exceeds the desired
maximum value. Additionally, problems may occur where the feedback
signal has a value that may cause uncontrollable modulation.
To remedy the potential problems with the feedback signal, the
present invention provides an apparatus and method for compensating
for phase shifts in an optical signal generated by an optical
device. Specifically, the apparatus of this embodiment includes an
array of detector assemblies 102 in optical communication with the
beam splitter 96 for receiving the interference signal created by
the combination of the reference and optical signals and generating
a feedback signal. Connected to the output of the array of detector
assemblies is a processor 104 for analyzing the lockin signal prior
to input of the lockin signal into the array of optical phase
modulators 94, such that the lockin signal is within the operation
parameters of the array of optical phase modulators.
With reference to FIG. 6, the operation of the apparatus of the
present invention to control the phase modulation of the laser
system is shown. In this embodiment of the present invention, the
plurality of detector assemblies receive respective portions of the
interference signal produced by the combination of the reference
and power signals. (See step 200). The plurality of detector
assemblies generate respective lockin signals representing the
difference in phase between the reference and power signals. (See
step 210). The processor receives the plurality of lockin signals
and compares the lockin signals to a predetermined set point value.
(See step 220). If any of the plurality of lockin signals exceed
the predetermined set point value, the processor combine the lockin
signals with a step waveform and generates a combined feedback
signal having a value within desired operation parameters of the
optical phase modulator. (See step 230). If none of the lockin
signals exceed the predetermined set point value, the lockin
signals are not altered. The feedback signals or the lockin
signals, if not altered, are next provided to the array of optical
phase modulators, where the feedback or lockin signals are used to
alter the phase of the output signal. (See step 240).
As discussed above, the present invention includes a plurality of
detector assemblies for receiving the interference signal and
generating a plurality of feedback signals representative of the
interference signal. With reference to FIG. 10, in one embodiment
of the present invention, the plurality of detector assemblies 102
each include a detector 106 and a lockin amplifier 108. In this
embodiment of the present invention, the detector for each detector
assembly receives the interference signal generated by the
interference of the reference and power signals. The lockin
amplifier for each detector assembly, in turn, generates a lockin
signal that is proportional to the sine of the phase difference
between the reference and power signals.
In this embodiment, the reference leg 84 of the present invention
further includes an array of reference optical phase modulators 110
upstream of the beam splitter 96 for phase modulating the reference
signal prior to interference of the reference and output signals.
The apparatus of this embodiment also includes either one or a
plurality of frequency generators 112 that are connected to both
the array of reference phase modulators 110 and the lockin
amplifiers 108. In this embodiment of the present invention, the
array of reference optical phase modulators dither the reference
signal at a reference frequency generated by the frequency
generator.
As discussed, the present invention combines the lockin signal with
a step waveform and generates a combined feedback signal having an
appropriate value for application to the array of optical phase
modulator. For instance, in one embodiment of the present
invention, the predetermined set point value may represent a
desired maximum value. The processor 104 of this embodiment
analyzes the lockin signals and combines the lockin signals with
step waveforms if they exceed the predetermined set point value. In
another embodiment, the predetermined set point value may represent
a range of lockin values, where lockin values outside this range
may cause uncontrollable changes in modulation. In this embodiment
of the present invention, the processor 104 analyzes the lockin
signals, and if the lockin signals are not within the range of
lockin signals defined by the predetermined set point values, the
processor combines the lockin signal with a step waveform and
generates a combined feedback signal having a value within desired
operation parameters of the optical phase modulator.
The above disclosure details the use of the present invention in
relation to two separate laser systems. It must be understood that
the present invention may be used with laser systems that use
feedback loops to control the phase of an optical signal output by
the laser system. Additionally, the present invention has been
illustrated with many different types of components. It must be
understood that these present invention is not limited to the
components shown, and the use of any components that perform
similar functions to the components listed above are
contemplated.
For instance, the master oscillator may be either a diode pumped
fiber laser, a single mode diode laser, or diode pumped rods, slabs
or mirrors. Additionally, the master oscillator preferably provides
a primary laser signal having a predetermined wavelength and a
predetermined power level. For example, the primary laser signal
may have a predetermined wavelength of 1064 nanometers and a
predetermined power level of 20 mW. However, the primary laser
signal can have other wavelengths and power levels without
departing from the spirit and scope of the present invention. In
addition, the master oscillator typically provides a primary laser
signal having a predetermined wavefront, such as TM.sub.00, and a
predetermined linewidth.
Similarly, the optical phase modulators may be implemented in
several ways. For example the optical phase modulators may be
liquid crystal modulators, electrooptic phase modulators, in-line
fiber optic phase modulators which are responsive to stress,
electric fields, magnetic fields or temperature,
microelectromechanical optics such as semiconductor wafers having
movable mirrors defined thereon or other electrically-actuated
phase modulators known to those skilled in the art.
The beam splitters implemented in the present invention can be
dichroic filters, partial transmission beam splitters, fiber optic
combiners, or integrated optic combiners. Further, the detectors
may be photo sensitive diodes or similar optical light detectors.
The optical fiber amplifiers of the present invention are
preferably optical fibers having a core doped with one or more rare
earth elements, such as ytterbium, neodymium, praseodymium, erbium,
holmium and thulium that amplify the optical signal when excited
with a pump.
Additionally, the processor of the present invention may consist of
any number of devices. The processor may be a data processing
device, such as a microprocessor or microcontroller or a central
processing unit that executes functions. The processor could be
another logic device such as an integrated communication processor
device, a custom VLSI (Very Large Scale Integration) device or an
ASIC (Application Specific Integrated Circuit) device.
In addition to providing apparatus and methods, the present
invention also provides computer program products for compensating
for phase shifts in an optical signal generated by an optical
device having an optical phase modulator for altering the phase of
the optical signal and a detector assembly that generates a lockin
signal representing an optical phase difference between the power
signal and a reference signal. The computer program products have a
computer readable storage medium having computer readable program
code means embodied in the medium. With reference to FIGS. 5 and
10, the computer readable storage medium may be part of the memory
device 114, and the processor, either 50 or 104, of the present
invention may implement the computer readable program code means
for controlling the phase of an optical signal output by a laser
system as described in the various embodiments above.
The computer-readable program code means includes first
computer-readable program code means for comparing the lockin
signal to at least one predetermined set point value. Further, the
computer-readable program code means also includes second
computer-readable program code means for combining the lockin
signal with a step waveform and generating a combined feedback
signal, if the lockin signal exceeds the set point value, such that
the combined feedback signal has a value within desired operation
parameters of the optical phase modulator.
With reference to the first computer-readable program code means,
as discussed previously with respect to the various apparatus and
methods of the present invention, the predetermined set point of
the first computer-readable program code means may represent
different set point values. For instance, in one embodiment, the
predetermined set point value may represent a desired maximum value
above which saturation may occur. In this embodiment, the second
computer-readable program code means will combine the lockin signal
with a step waveform to thereby generate a combined feedback signal
if it exceeds the predetermined set point value. In another
embodiment, the predetermined set point value may represent a range
of set point values, where lockin values outside the range may
cause uncontrollable modulation changes. In this embodiment, the
second computer-readable program code means will combine the lockin
signal with a step wave form if the lockin signal is not within the
range of lockin values defined by the predetermined set point
values.
With reference to the second computer-readable program code means,
as discussed previously with respect to the various apparatus and
methods of the present invention, the second computer-readable
program code means may combine the lockin signal with an
exponential step waveform, if the lockin signal exceeds the
predetermined set point value. Specifically, in one embodiment of
the present invention, the second computer-readable program code
means may combine the lockin signal with an exponential step
waveform having a step portion of 2.pi.n, where n is an
integer.
In this regard, FIGS. 5, 6, and 10 are a block diagram, flowchart
and control flow illustration of methods, systems and program
products according to the invention. It will be understood that
each block or step of the block diagram, flowchart and control flow
illustration, and combinations of blocks in the block diagram,
flowchart and control flow illustration, can be implemented by
computer program instructions. These computer program instructions
may be loaded onto a computer or other programmable apparatus to
produce a machine, such that the instructions which execute on the
computer or other programmable apparatus create means for
implementing the functions specified in the block diagram,
flowchart or control flow block(s) or step(s). These computer
program instructions may also be stored in a computer-readable
memory that can direct a computer or other programmable apparatus
to function in a particular manner, such that the instructions
stored in the computer-readable memory produce an article of
manufacture including instruction means which implement the
function specified in the block diagram, flowchart or control flow
block(s) or step(s). The computer program instructions may also be
loaded onto a computer or other programmable apparatus to cause a
series of operational steps to be performed on the computer or
other programmable apparatus to produce a computer implemented
process such that the instructions which execute on the computer or
other programmable apparatus provide steps for implementing the
functions specified in the block diagram, flowchart or control flow
block(s) or step(s).
Accordingly, blocks or steps of the block diagram, flowchart or
control flow illustrations support combinations of means for
performing the specified functions, combinations of steps for
performing the specified functions and program instruction means
for performing the specified functions. It will also be understood
that each block or step of the block diagram, flowchart or control
flow illustrations, and combinations of blocks or steps in the
block diagram, flowchart or control flow illustrations, can be
implemented by special purpose hardware-based computer systems
which perform the specified functions or steps, or combinations of
special purpose hardware and computer instructions.
Many modifications and other embodiments of the invention will come
to mind to one skilled in the art to which this invention pertains
having the benefit of the teachings presented in the foregoing
descriptions and the associated drawings. Therefore, it is to be
understood that the invention is not to be limited to the specific
embodiments disclosed and that modifications and other embodiments
are intended to be included within the scope of the appended
claims. Although specific terms are employed herein, they are used
in a generic and descriptive sense only and not for purposes of
limitation.
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